Because it's not his fault. Subconscious awareness and even resistance is not culpability because otherwise it's not fair - how can you condemn someone for what they can't control!
I agree that it isn't his fault, but the idea of resistance
is problematic. Spike stalked killed and turned up to a dozen humans which means that 'he' (the trigger part of him) was thinking,
targeting and interacting with his victims, luring or conning or seducing or whatever. That isn't some zombie mind-control, that's a thinking agent of the First (hence the 'Sleeper' title).
You start speculating 'well, maybe he had enough wherewithal to control
where he killed his victims and it begins to get murky. He could control where, he wanted to be stopped but couldn't, except that he could stop with Buff... so hey, maybe he could've tried harder then? Maybe he could've left Buffy a subconscious clue? Maybe he could select a particular victim? Instead of where it was who?
Maybe he targeted Holden because that guy is really chatty and insightful and knew Buffy and then maybe Holden would reveal who sired him. Maybe that was Spike's plan all along...
Unfortunately this is all speculation. One thing could lead to another and could lead to another and it becomes a Spike who is maybe can choose some stuff and therefor is both victim and victimizer.
Luckily I don't believe any of that because there's no in-show evidence to show it, in my view.
Where is
could see some subconscious resistance is
after the trigger is revealed and suddenly it turns Spike into a rage-monster, much different than before. All the predatory-ness have gone and he's just the demon lashing out. One could see this as Spike fighting the trigger (although I doubt it for various reasons).
Again, because this is S7 writing there's no actual interesting reveal to mull over so its just some weird bit of discontinuity.
Also blood breaking mind control isn't unprecedented with Dracula something within his is what she reacts to breaking his hold over her.
Yeah, I mentioned this, the whole blood tasting thing. Not just with Drac but also way back in Prophecy Girl against The Master. Something about blood-tasting makes the mind-control thing go all wonky on both sides.
It's also a possibility the First wants Buffy to find out and kill him.
I'm not sure this makes sense since trigger!Spike is a massive asset to The First. Buffy just killing Spike seems like a waste of a resource. In fact it worked to its advantage that Spike, even when discovered, was brought into Buffy's group of allies with his trigger still intact.
Like a dagger aimed at the heart of the scoobies that Buffy willingly put there, but that's another story.